This invention is directed to a process for producing cationic epoxy resins carrying oxazolidine groups suitable for use as paint binders, particularly for the formulation of cathodically depositable paints (CED-paints), to the paint binders produced, and to paints utilizing the binders.
Various methods are known, as disclosed in EP-B1-00 28 401, EP-B1-00 28 402, EP-A2-96 768 and AT-PSS Nos. 375 952, 374 816, 375 946 and 376 231, for producing oxazolidine group carrying compounds and for reacting the compounds as semiesters or monourethanes formed through reaction with dicarboxylic acid anhydrides or with diisocyanates with epoxy resins either via the hydroxy groups or functional groups of the ring. Through the introduction of such basic groups the solubility of binder materials at a relatively low degree of neutralization can be essentially improved. Furthermore, due to the great variety of possible substituents, the paint and film properties can be tailored for particular applications. However, heretofore it was not possible to obtain binders, water dilutable upon neutralization, by direct addition of such hydroxy or ring functional oxazolidine compounds to epoxy resins, since such compounds immediately led to a crosslinking of the epoxy resins and, thus, to gelling of the reaction batch. [As used herein, the term "ring functional oxazolidine compounds" is understood to embrace five-membered ring systems carrying the structure ##STR2## wherein R is H or an alkyl or aryl radical without alcoholic hydroxy functionality. The preparation of compounds of this type is described in AT-PS No. 375 946.] A direct addition of the reaction partners is, however, desirable for water-dilutable binders since, on the one hand, the number of saponifiable ester linkages is reduced; and, on the other hand, working with isocyanates and the preparation of intermediates can be avoided.